1. Field of the Invention
The present invention relates to a polarizing liquid crystal film and a method for manufacturing the same. More particularly, the present invention relates to a polarizing liquid crystal film having no alignment layer and a method for manufacturing the same, a light guide plate integrated with the polarizing liquid crystal film, and a backlight unit.
2. Description of the Related Art
In general, the application range of a liquid crystal display (“LCD”) has been extended because LCDs possess good lightweight, thin, low-power drive, full color and high resolution characteristics. LCDs have been currently used in computers, notebook computers, PDAs, telephones, TVs, audio/video devices, and the like. Such LCDs are used to adjust an amount of light transmitted in accordance with image signals applied to a number of control switches arrayed in a matrix form and then display a desired image on an LCD panel. Since the LCD cannot emit light by itself, it requires a light source such as a backlight unit.
FIG. 1 is an exploded perspective view of a general backlight unit for an LCD, which corresponds to an edge-type backlight unit. Referring to FIG. 1, the backlight unit includes a lamp 1, a reflection plate 2, a light guide plate 3, a diffusion plate 4, a first prism sheet 5, a second prism sheet 6, and a brightness enhancement film 7. An LCD panel 8 is disposed on the top of the backlight unit. Visible light emitted from the lamp 1 travels to the light guide plate 3 having an inclined bottom surface, and the light is then guided toward a top surface of the light guide plate 3. At this time, the light passing through the top surface of the light guide plate 3 is emitted not only vertically but also at a variety of angles from the top surface of the light guide plate 3. The diffusion plate 4 diffuses light incident from the light guide plate 3 to prevent the light from being partially focused. Each of the first and second prism sheets 5 and 6 is formed with triangular prisms arrayed in a certain direction on a surface thereof, and the prisms of the first and second prism sheets 5 and 6 are arranged to extend in directions which cross each other. The first and second prism sheets 5 and 6 serve to focus the light diffused from the diffusion plate 4 in a direction perpendicular to a plane of the LCD panel 8.
The brightness enhancement film 7 is a film for enhancing brightness and providing a wide light viewing angle by reducing the loss of light emitted from the prism sheets 5 and 6. A dual brightness enhance film (“DBEF”) or diffuse reflective polarizer film (“DRPF”) sheet is generally used as the brightness enhancement film. The DBEF or DRPF sheet is a reflective polarizing film taking the shape of a thin film and prevents light from being absorbed into a bottom polarizing plate of the LCD panel 8. The DBEF or DDRPF sheet also serves to enhance the brightness of the LCD panel 8 by allowing the light, which will be absorbed and vanished in this manner, to be reused.
In the case of a conventional backlight unit, since a plurality of optical sheets are arranged on the top of a light guide plate, the total thickness of a backlight unit is inevitably increased. Further, since light incident from a light source passes through the plurality of optical sheets and then propagates to an LCD panel, there is a problem in that the loss of light occurs. Furthermore, there is another problem in that the unit cost of production is increased since the DBEF and DRPF sheets are very expensive.
FIG. 2A is a sectional view of a conventional polarizing liquid crystal film, and FIG. 2B is a sectional view of a light guide plate with the polarizing liquid crystal film of FIG. 2A attached thereto.
As shown in FIG. 2A, if liquid crystal polymers (“LCPs”) with a high refractive index, which causes a polarization effect, are aligned on the top of a light guide plate, the brightness of a display can be enhanced without using an expensive brightness enhancement film.
Referring again to FIG. 2A, the polarizing liquid crystal film 20 includes a TriAcetyl Cellulose(“TAC”) sheet 22 serving as a protection film, an alignment layer 24 disposed on the TAC sheet 22, and liquid crystal 26 aligned on the alignment layer 24. FIG. 2B shows a light guide plate integrated with a polarizing liquid crystal film in which the polarizing liquid crystal film 20 of FIG. 2A is attached to a light guide plate 30. At this time, the liquid crystal 26 of the polarizing liquid crystal film 20 is arranged on a surface of the light guide plate 30 from which light is emitted. The polarizing liquid crystal film 20 prevents light from being absorbed into the bottom polarizing plate of the LCD panel by polarizing predetermined light components to be vanished.
FIGS. 3A to 3D are sectional views illustrating a process of manufacturing the conventional polarizing liquid crystal film and a light guide plate integrated with the polarizing liquid crystal film. Referring to FIGS. 3A to 3D, a TAC sheet 22 is first provided as a protection film, as shown in FIG. 3A. Then, a thin alignment layer 24 for aligning liquid crystal is formed on the TAC sheet 22, as shown in FIG. 3B. Since the liquid crystal alignment is easily broken due to a surface friction force on the TAC sheet 22, the liquid crystal 26, as described with respect to FIG. 3C, cannot be aligned directly on the TAC sheet 22. Therefore, the thin alignment layer 24 is formed as described above. Further, the thin alignment layer 24 is formed extremely thin at a thickness of about several angstroms (Å). At this time, the thin alignment layer 24 is mainly made of polyimide. Then, the thin alignment layer 24 is rubbed in a single direction such that the liquid crystal 26 is aligned on a surface of the alignment layer 24 in a certain direction. Next, the liquid crystal 26 is applied on the thin alignment layer 24, and thus, the polarizing liquid crystal film is manufactured, as shown in FIG. 3C.
After the manufactured polarizing liquid crystal film 20 is disposed on the top of the light guide plate 30, a compression process is executed. Thus, a light guide plate integrated with a polarizing liquid crystal film is formed, as shown in FIG. 3D. At this time, the liquid crystal 26 of the polarizing liquid crystal film 20 is arranged on the top of the light guide plate 30 which corresponds to a light exiting surface of the light guide plate 30.
As described above, a process of forming a thin alignment layer on a TAC sheet is needed to manufacture a polarizing liquid crystal film. To this end, however, extremely various and complex processes are performed. Therefore, there is a problem in that the unit cost of production is increased and the productivity and efficiency are decreased.